Rainwater Collection Calculator

You can use this useful rainwater collection calculator to estimate the rainwater harvesting potential within a calendar year based on monthly historical rainfall data. The rainfall collection calculator uses the National Climatic Data Center's (NCDC) 30-year average rainfall data for the area of interest.

Rainwater Collection Calculator




Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Average Rainfall (inches)
Rainwater Usage (gallons)

Note: The average monthly precipitation values applied within the calculations represent a historical overview of the latest 30-year period of data that is available. There is no guarantee that this level of precipitation will be observed in a given location during a specific month.

Rainfall Collection Equation

Computing the amount of rainfall in gallons involves inputting variables related to where you live and the size of the given rainwater harvesting system.

You can calculate the amount of rainwater harvested by using the following equation:

V (gal.) = CA (sq.ft.) × P (in.) × RC × 0.623


V is the volume of rainfall harvested (gallons),

CA is the catchment area (the area from which the rain is collected) in square feet,

P is the depth of the rainfall in inches (average monthly, annual, etc.),

0.623 is the conversion factor,

RC is the runoff coefficient (expressed as a decimal).

The runoff coefficient variable relates to the amount of water that drains free of the surface as a ratio of the total amount that falls on the surface. For instance, if you have a runoff coefficient of 0.9, of the total volume of rain that fell on the catchment surface, 10% remained on the surface, while 90% drained off.

The table below presents an overview of the runoff coefficients for the various types of catchments.

Runoff Coefficients for Various Types of Catchments
Type of Catchment Runoff Coefficient (RC)
Galvanized iron sheets > 0.9
Corrugated metal sheets 0.7 – 0.9
Tiles 0.8 – 0.9
Concrete 0.6 – 0.8
Brick pavement 0.5 – 0.6
Rocky natural catchments 0.2 – 0.5
Soil on slopes less than 10% 0.0 – 0.3
Green area 0.05 – 0.1

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